1. Field of the Invention
The present invention relates to an electron beam exposing apparatus and, more particularly, to a variable shaping electron beam exposing apparatus.
2. Description of the Prior Art
It has been recent practice to expose a resist to light using an electron beam for ultrafine processing for forming the planar shape of an LSI in the resist. A pattern in generally formed on the resist by scanning an electron beam converging to a spot. However, a variable shaping electron beam exposing apparatus has been proposed to form a drawn image with an electron beam in advance and to project the image on the resist for improving the exposing speed. With this apparatus, the electron beam irradiated from an electron gun is shaped by two shaping plates which have square openings and which are superposed on each other with their axes shifted from each other. The image at the openings of the shaping plates will be referred to as an aperture image hereinafter. For drawing a distinct pattern on the resist, this device must satisfy the following three requirements. First, the spread of the electron beam due to the space-charge effect or the Belcher effect must be kept under a certain value so that the pattern will not be blurred. The spread of the electron beam and the luminous intensity of the aperture image are inversely proportional to each other. The second requirement is to increase the luminous intensity as much as possible while still keeping the spread of the electron beam below a certain value. Thirdly, the current density in the variably shaped aperture image must be uniform so that the drawin image will not vary in density. The third requirement amounts to requiring that the opening of the first shaping plate be uniformly irradiated by the electron beam from the electron gun.
The first requirement can be satisfied to a certain extent by the geometrical construction of the apparatus. The electronic optical system is generally so constructed that the aperture image is reduced in scale with a plurality of lenses, the electron beam is spread so as to gradually widen the crossover image (image at the focal point of the electron beam irradiated from the electron gun), and the electron beam is focused with an objective lens with a great angular aperture so as to form a small aperture image on the sample. With such a construction, the transmission of the electron beam becomes maximum, and the space-charge effect may be reduced to the minimum.
With a conventional tungsten cathode used in an electron microscope, the service life becomes as short as several tens of hours when the luminous intensity is made greater, and the electron beam intensity may not be made sufficiently great when the spread of the electron beam is suppressed to be below a certain value. Thus, the electron gun having a tungsten cathode is not suitable for an electron beam exposing apparatus of high precision and high luminance.
An electron gun having a single-crystal cathode, especially a lanthanum hexaboride (LaB.sub.6) cathode, has been proposed as an electron gun which has a long service life and which provides a sufficient luminance. However, an electron beam exposing apparatus using an LaB.sub.6 electron gun which satisfies the two requirements described above, that is, which can draw a uniform image without blurring, has not been realized.